Allometry of external morphology and sexual dimorphism in the red porgy (Pagrus pagrus)

Body weight (W), nine morphometric variables and total length (TL) were estimated for each sex in the protogynous red porgy, (Pagrus pagrus) reared in captivity. The power function Y=aX(b) was used to evaluate growth-related changes in morphology. The first derivative, dY/dTL=abTL(b-1), where Y is each morphometric character, was used to study the growth pattern of each character relative to growth in length (TL). The length-weight relationship showed isometric growth and no inter-sex variability. The analysis of the morphometric variables demonstrated the existence of a substantial degree of differences between the sexes. Males showed an isometric growth pattern (I) for head height and an accelerated (+A) or retarded (-A) growth pattern for the pre-orbital and post-orbital distance, respectively. However, females showed a retarded (-A) growth rate of the head height and an isometric growth pattern (I) for pre-orbital and post-orbital distances. Thus, the male red porgy appears to have a taller head compared to females. Additionally, the head in males is wider and the orbits are positioned more posteriorly in the head. The adaptive significance of these differences remains currently unclear but should be studied in relation to protogynous hermaphroditism and the specific features of male biology in the red porgy.